<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
<input type="text" id="i1">
<input type="text" id="i2">
<input type="button" value="加" id="b1" onclick="f(1)">
<input type="button" value="减" id="b2" onclick="f(2)">
<input type="button" value="乘" id="b3" onclick="f(3)">
<input type="button" value="除" id="b4" onclick="f(4)">
<h3>
  结果：<span></span>
</h3>
</body>
<script>
let i1 = document.querySelector("#i1")
let i2 = document.querySelector("#i2")
let s = document.querySelector("span")
// let num1 = i1.value  ----不行
// let num2 = i2.value
function f(x) {
    if (isNaN(i1.value) || isNaN(i2.value)) {
        s.innerText = "输入错误，笨蛋！！"
        return;
    }
    switch (x) {
        case 1:
            s.innerText = i1.value*1+i2.value*1;
            break;
        case 2:
            s.innerText = i1.value-i2.value;
            break;
        case 3:
            s.innerText = i1.value*i2.value;
            break;
        case 4:
            s.innerText = i1.value/i2.value;
            break;
    }
}
function sum(num1,num2) {
    if (isNaN(num1) || isNaN(num2)) {
        s.innerText = "输入错误，笨蛋！！"
        return;
    }
    s.innerText = num1*1+num2*1;
}
function sub(num1,num2) {
    if (isNaN(num1) || isNaN(num2)) {
        s.innerText = "输入错误，笨蛋！！"
        return;
    }
    s.innerText = num1-num2
}
function mul(num1,num2) {
    if (isNaN(num1) || isNaN(num2)) {
        s.innerText = "输入错误，笨蛋！！"
        return;
    }
    s.innerText = num1*num2
}
function abs(num1,num2) {
    if (isNaN(num1) || isNaN(num2)) {
        s.innerText = "输入错误，笨蛋！！"
        return;
    }
    s.innerText = num1/num2
}
</script>
</html>